Stabilized ammonium nitrate propellant



with a matrix former or binder. sitions must produce gas at a uniformrate; therefore, to

the burning of the ammonium nitrate.

d S P O Umte tates atent, Ice meme, Dec,

other elements in addition to carbon and hydrogen, for 3,113,957example, as in Thiokol rubber and neoprene. The stoi- STABIUZED AMMDNIUMNITRATE chiometry of the composition is improved, with respectPROPELLANT Walter W. Butcher, South Bend, Ind, assignor to Standand OilCompany, Chicago, Ill., a corporation of Indiana No Drawing. Filed June30, 196i), Ser. No. 40,161

6 Claims. (Cl. 14919) This invention relates to gas-generatingcompositions or propellants which are based upon ammonium nitrate as theoxidizer.

the grain there is added a catalyst adapted for promoting When thebinder contains a cellulose ester such as cellulose acetate as a Ws-major component, the ammonium nitrate composition suffers thedisability of instability at elevated temperatures.

The Armed Forces of necessity must store gas-generators and propellantsover the entire surface of the earth. It is common for storage buildingsin the tropics to reach temperatures of 150 F. In order'to meet theseconditions, the Armed Forces require ammonium nitrate propellants to bestorage stable at temperatures on the order of 170 F. for a period ofone year. After prolonged storage at these temperatures, celluloseester-containing ammonium nitrate propellants tend to evolve gas; inextreme cases the gas evolution is sufiicient to produce fissures in thegrain and even to break the grain into several pieces.

Gas-generating or propellant compositions based upon ammonium nitrate asthe oxidizerv basically comprise ammonium nitrate, an oxidizable organicbinder material, a catalyst adapted for promoting the burning rate ofthe composition, and a gas evolution stabilization additive adapted forincreasing the storage stability of the composition at elevatedatmospheric temperatures. It has now been discovered that salts ofnitrilotriacetic acid derived from a nitrogen base are effective toimprove the storage stability of such compositions.

The improved stabilizer composition of the invention contains ammoniumnitrate as the major component. The ammonium nitrate may be ordinarycommercial ammonium nitrate such as is used for fertilizers. Thiscommercial grade material contains a small amount of impurities and theparticles are usually coated with moisture resisting material such asparaflin wax. Military grade ammonium nitrate which'is almostchemically'pure is particularly suitable. The ammonium nitrate ispreferably in a finely divided particulate form which may be eitherproduced by prilling or by grinding. The ammonium nitrate is the majorcomponent of the gas-generator composition and usually the compositionwill contain between about 55 to 80 percent of ammonium nitrate. (It isto be understood that all percentages set out herein are percent byweight of the total composition.)

In order to permit the shaping of the ammonium nitrate composition intodefinite configurations, a matrix former or hinder material is present.When ammonium nitrate decomposes, free-oxygen is formed. Advantage ofthe existance of this free-oxygen istaken, and oxidizable organicmaterials are used as the binders. These oxidizable organic materialsmay contain only carbon and hydrogen,

for example, high molecular weight hydrocarbons such as asphalts orresiduums, and rubbers, either natural or synthetic. Or, the oxidizableorganic material may contain Gas-generator compoto smoke production, bythe use of oxygenated organic materials as the binders. The binder ormatrix former may be a single compound such as a rubber or asphalt, orit may be a mixture of compounds. The mixtures are particularly suitablewhen special characteristics are to be imparted to the grain whichcannot be obtained by the use of a single compound.

The multi-component binder or matrix former commonly consists of apolymeric base material and a plasticizer therefor. Particularlysuitable polymeric base materials are cellulose esters of alkanoic acidscontaining from 2 to 4 carbon atoms such as cellulose acetate, celluloseacetate butyrate and cellulose propionate; the polyvinyl resins such aspolyvinylchloride and polyvinyl acetate are also good bases; styreneacrylonitrile is an example of a copolymer which forms a good basematerial. In general, the binder contains between about 15% and 45% ofthe particular polymeric base material.

The plasticizer component of the binder is broadly defined as anoxygenated hydrocarbon. The hydrocarbon base may be aliphatic oraromatic or may contain both forms. The oxygen may be present in theplasticizer in either linkage and/or hydroxyl group and/or carboxylgroups; also the oxygen may be present in inorganic substituents,particularly nitro groups. In general, any plasticizer which is adaptedto plasticize the particular polymer may be used in the invention.Exemplary classes of plasticizers which are suitable are set out below.

It is to be understood that these classes are illustrative only and donot limit the types of oxygenated hydrocarbons which may be used toplasticize the polymer.

Di-lower alkyl-phthalates, e.g., dimethyl phthalate, dibutyl phthalate,dioctyl phthalate and dimethyl nitrophthalate.

Nitrobenzenes, e.g., nitrobenzene, dinitrobenzene, nitrotoluene,dinitrotoluene, nitroxylene, and nitrodiphenyl.

Nitrodiphenyl ethers, e.g., nitrodiphenyl ether and 2,4-

dinitrodiphenyl ether.

Tri-lower alkyl-citrates, e.g., triethyl citrate, tributyl citrate andtriamyl citrate.

Acyl tri-lower alkyl-citrates where the acyl group contains 2-4 carbonatoms, e.g., acetyl triethyl citrate and acetyl tributyl citrate.

Lower alkylene-glycol oxolates, e.g., diethylene glycol oxoiate andpolyethylene glycol (200) oxolate.

Lower alkylene-glycol maleates, e.g., ethylene glycol maleate andbis-(diethylene glycol monoethyl ether) maleate.

Lower alkylene-glycol diglycollates, e.g., ethylene glycol diglycollateand diethylene glycol diglycollate.

Miscellaneous diglycollates, e.g., dibutyl diglycollate, dilmethylalkyldiglycollate and methyl Carbitol diglycolate.

Lower alkyl-phthalyl-lower alkyl-glycollate, e.g., methyl phthalyl ethylglycollate, ethyl phthalyl ethyl glycollate and butyl phthalyl butylglycollate.

Di-lower alkyloxy-tetraglycol, e.g., dimethoxy tetra glycol and dibutoxytetra glycol.

Nitrophenylether of lower alkylene glycols, e.g., dinitrophenyl ether oftriethylene glycol and nitrophenyl ether of polypropylene glycol.

Nitrophenoxy alkanols wherein the alkanol portion is derived from aglycol having a molecular weight of not more than about 200. These maybe pure compounds or admixed with major component bis(nitrophenoxy)alkane.

A single plasticizer may be used or more usually two or moreplasticizers may be used in conjunction. The particular requirementswith respect to use will determine not only the polymer but also theparticular plasticizer or combination of plasticizers which are used.

The mixture of ammonium nitrate, polymeric base and oxygenatedhydrocarbon is essentially as insensitive to shock as is ammoniumnitrate itself. It is extremely difficult to get this particular mixtureto burn. Smooth burning is attained by the addition of a catalyst to themixture. This catalyst is distinguished from the well-known sensitizers.For example, nitro starch or nitroglycerine may be added to ammoniumnitrate in order to increase its sensitivity to shock and enable it tobe more easily detonated for explosive use. Catalysts as a class do notpromote sensitivity and are used to cause the ammonium nitratecomposition to burn for example like a cigarette. The effectiveness ofthe catalyst is in general measured by its ability to impart a finiteburning rate to a strand of ammonium nitrate composition. The burningrate is specified as inches per second at a given pressure andtemperature; usually these burning rates are obtained by a bombprocedure operating at 1000 p.s.i. and about 75 F. temperature.

Many catalysts which promote the burning of ammonium nitratecompositions are known. The inorganic chromium salts form the best knownclasses of catalysts. The better known members of this class areammonium chromate, ammonium polychromate, the alkali metal chromates andpolychromates, chromic oxide, chromic nitrate, and copper chromite.Ammonium dichromate is the most commonly used chromium salt. Varioushydrocarbon amine chromates such as ethylene diamine chromate andpiperidine chromate are also excellent chromium catalysts. Certain heavymetal cyanides, particularly those of cobalt, copper, lead, nickel,silver and zinc are effective catalysts. The cyanamides of barium,copper, lead mercury and silver are effective catalysts. The variousPrussian blues are excellent catalysts.

In addition to the above primarily inorganic catalysts, various organiccatalysts are known. The organic catalysts are particularly useful whenit is desired to have combustion products which are gases or vapors andthereby do not erode gas exit orifices. Two catalysts which do notcontain any metal components are pyrogene blue (Color Index 95696l) andmethylene blue. Particularly suitable catalysts are the alkali metalbarbiturates.

The chromium salts and Prussian blue promote the rate of gas evolutionof ammonium nitrate compositions containing cellulose esters andtherefore the gas evolution prevention additive of the invention isparticularly applicable to compositions containing these catalysts.

The catalysts are present in the composition in an amount determined byits use and also by the particular catalyst. In general between about 1and 12 percent of catalyst is present and more usually between about 2and 4 percent.

In addition to the basic components, i.e., ammonium nitrate, binder andcatalyst, the composition may contain other materials. For example,materials may be present to improve low temperature ignitability, forinstance oximes may be present or asphalt may be present. Surfactantsmay be present in order to improve the coating of the nitrate with thebinder and to improve the shape characteristics of the composition.Various burning rate promoters, such as finely divided carbon, which arenot catalyst per se, may also be present.

It has been discovered that a composition containing ammonium nitrate, apolymeric base, particularly a cellulose ester, an oxygenatedhydrocarbon adapted to plasticize the polymeric base, and a burning ratecatalyst may be effectively stabilized against gas evolution duringstorage at elevated atmospheric pressures by the addition of a salt ofnitrilotriacetic acid, the salt being derived by reacting a nitrogenbase with the acid. Illustrative examples of suitable nitrogen basesfrom which a salt may be made are ammonia; ammonium hydroxides;hydrazine; quaternary ammonium hydroxides, such as tetramethyl ammoniumhydroxide, tetraethyl ammonium hydroxide, tetrapropyl ammoniumhydroxide, ethyl trimethyl ammonium hydroxide, and phenyl trimethylammonium hydroxide; the aliphatic amines, such as the primary, secondaryand tertiary amines have methyl, ethyl, propyl, isopropyl, butyl,isobutyl, pentyl and hexyl radicals, e.g., methyl amine, methyl ethylamine, triisopropyl amine; naphthenic amines, such as cyclohexyl amine(hexahydroaniline); the aromatic amines, such as phenyl amine (aniline)diphenyl amine, toluidine, xylidine, and mesidine; mixedaliphatic-aromatic amines, such as phenyl methyl amine and tolyl ethylamine, wherein both the aliphatic and aromatic radicals are adapteddirectly to the nitrogen atom; and also mixed aliphatic-aromatic amines,such as benzyl amine, where the aromatic nucleus is separated from thenitrogen atom by an alkyl group; heterocyclic amines, such as pyridineand quinoline; and aliphatic, naphthenic and aromatic diamines, such asethylene diamine, hexamethylene diamine and benzidine (4,4'-biphenyldiamine). Ammonium hydroxide and hydrazine are particularly suitablenitrogen bases from which to form a salt of ni-trilotriacetic acid. Itis desirable that the salt used give a neutral or somewhat alkaline pHwhen dissolved in water.

The salts of nitrilotriacetic acid may be prepared by mixing in anaqueous solution the acid with the desired nitrogen base, evaporatingthe liquid therefrom, and drying the resulting salt. Evaporating anddrying temperatures of not higher than about 300 F. are preferred, inorder that the salt not decompose. The salts also may be prepared bymixing a nitrogen base with nitrilotriacetic acid in an organic solvent,such as an alcohol, and filtering and drying the resultant precipitant.

The amount of the above-described salt to be used in an ammonium nitratecomposition is determined by the instability of the particularcomposition or by the specific requirements for a particularcomposition. Increasing the specified storage time or the specifiedtemperature at which the composition must remain stable increases theamount of the salt to be used in the composition. In general, increasingthe amount of burning catalyst in the composition requires an increasein the amount of salt necessary to maintain constant gas evolutioncharacteristics. Normally, the amount of salt used will be between about0.1 and about 10 weight percent, more normally between about 0.5 and 3percent. Various catalysts affect the gas evolution rate to differingdegrees.

The above-described salt may also be used in conjunction with otherknown gas evolution stabilization additives, such as aromatic amines,e.g., toluene diamine, diamine phenyl amine, naphthalene, and toluenetriamine, and/or a morpholine, such as N-phenylmorpholine, or amorpholine having an alkyl-substituted phenyl group, such asN-tolylmorpholine. In compositions which are particularly difiicult tostabilize, use of both an aromatic amine and a phenylmorpholine togetherwith an abovedescribed salt is advantageous. The amounts of theseadditional gas evolution stabilization compounds to be used rangebetween about 0.1 and percent, advantageously 0.5 to 2 percent.

It is to be noted that the aromatic amine used as a supplemental gasevolution stabilization additive may be the same compound, or of thesame structural class of compounds, as the nitrogen base used to preparethe above-described salt, e.g., toluene diamine.

Broadly the composition will contain between about 20 and 35 weightpercent of binder when the polymeric base material is a cellulose esterof an alkanoic acid containing 2 to 4 carbon atoms and an oxygenatedhydrocarbon plasticizer therefor. A particularly useful compositionconsists of cellulose acetate, about 6l2%; acetyltriethylcitrate, about6-l2%; a mixture having two to four parts of dinitrophenoxyethanol toone part of bis(dinitrophenoxy)ethane, about 6l2%; carbon, about 24%;toluene diamine, about /2 N-phenylmorpholine, about /2 an ammonium saltof nitrilotriacetic acid, about 2 4%; and catalyst, about 24%.

TESTS Two ammonium nitrate-type compositions were tested for burningrate, storage stability, and other characteristics required by militaryspecifications.

Each composition was prepared by mixing together for one hour in alaboratory mixer a 300-gram batch having ingredients in the proportionsindicated in the table set forth below. The mixing temperature was about100 C. Lacquer grade commercial cellulose acetate analyzing about 55percent of acetic acid equivalent was the polymer base. Two plasticizerswere used. One plasticizer contained about three parts of dinitrophenoxyethanol and one part of bis(dinitrophenoxy)ethane, obtained by thereaction of dinitrochlorobenzene and ethylene glycol in the presence ofaqueous sodium hydroxide solution. The other plasticizer was acetyltriethyl citrate. Both compositions contained about 3% of sodiumbarbiturate as a burning rate catalyst, and about /2% each of toluenediamine and N-phenylmorpholine. Composition B contained in additionabout three weight percent of an ammonium salt of nitrilotriacetic acidas a gas evolution stabilization additive.

The ammonium salt of nitriloacetic acid used as a gas evolutionstabilizer in Composition B was prepared by adding ammonium hydroxide tonitrilotriacetic acid until a pH of 7 was obtained, evaporating thesolution to dryness, and drying the resulting salt in an oven at about250 F. An aqueous solution of the salt had a pH of essentially 7.

After mixing the resulting pasty mass was compression molded into a slabapproximately one-half inch in thickness. The slab was subsequentlysawed into strips for the burning rate test and broken into smallerpieces for the storage stability test.

The burning rate tests were conducted in a Crawford bomb pressured at1000 p.s.i.g. and 25 C.

The high temperature storage stability or the compositions wasdetermined in a laboratory test as follows. A small sample, about threegrams, of the composition was placed in a vessel connected by tubing toa mercury manometer system which was so arranged that differentialreadings of the manometer were translatable into volume changes in thesystem. Since volume change of the composition sample itself can bedisregarded, the volume change in the system corresponds to the amountof gaseous decomposition products from the sample. The vessel wasinserted into a metal block provided with electrical heating elementsand controls which permit the b oc t b a nta ned a a empe ature of 150C. A

period of 15 minutes was allowed for the sample to come to thetemperature of C., at which time the manometer was zeroed. While thesample was maintained at 150 C., the time (induction period) wasmeasured between the zeroing of the manometer and the time when the gasevolution rate reached one cc. per gram per hour. Also the time wasmeasured between the zeroing of the manometer and the time at which agas evolution rate of five cc. per gram per hour was reached.

The foilowing ta'ole summarizes the ingredients of each compositiontested and the results of the burning rate and high temperaturestability tests conducted thereon:

Table I Composition N-phenyl morphol Ammonium salt of nitriloaceticac1:l

Totals Test Results:

Burning Rate, inches/see Pressure Exponent... Induction Period, hoursTine to gas evolution rate of 5 ce./g./hr., hours Thus having describedthe invention, what is claimed l. A composition consisting essentiallyof (a) ammonium nitrate as the predominant component, (b) an oxidizableorganic binder material wherein said binder material consistsessentially of a polymeric base selected from the class consisting ofcellulose esters of alkanoic acids containing from two to four carbonatoms, polyvinyl chloride, polyvinyl acetate and styrene-acrylonitrileand an oxygenated hydrocarbon adapted to plasticize said polymer, (0) acatalyst adapted for promoting the burning of said ammonium nitrate, and(d) a salt of nitrilotriacetic acid, said salt being derived from anitrogen base and being used in an amount at least suflicient to improvethe stability of said composition with respect to gas evolution.

2. The composition of claim 1 wherein said salt is an ammonium salt ofnitrilotriacetic acid.

3. The composition of claim 1 wherein said salt is derived fromhydrocarbon amines having between one and about ten carbon atoms permolecule.

4. The composition of claim 1 wherein said salt is derived fromhydrazine.

5. The composition of claim 1 wherein there is included between aboutone-half to about five weight percent of a compound selected from theclass consisting of aromatic hydrocarbon amines, N-phenylmorpholine andmixtures thereof.

6. A composition consisting essentially of (a) ammonium nitrate, (b)cellulose acetate, about 6l2%, (c) acetyl triethyl citrate, about 642%,(d) about 6-12% of an about 2:1 mixture of dinitrophenoxyethanol andbis- (dinitrophenoxy)ethane, (e) carbon, about 2-4%, (1) sodiumbarbiturate catalyst, about 24%, (g) toluene diamine, about /2 (h)N-phenylmorpholine, about /2%, and (i) an ammonium salt ofnitrilotriacetic acid, about 24%,

No references Gi t dv

1. A COMPOSITION CONSISTING ESSENTIALLY OF (A) AMMONIUM NITRATE AS THEPREDOMINANT COMPONENT, (B) AN OXIDIZABLE ORGANIC BINDER MATERIAL WHEREINSAID BINDER MATERIAL CONSISTS ESSENTIALLY OF A POLYMERIC BASE SELECTEDFROM THE CLASS CONSISTING OF CELLULOSE ESTERS OF ALKANOIC ACIDSCONTAINING FROM TWO TO FOUR CARBON ATOMS, POLYVINYL CHLORIDE, POLYVINYLACETATE AND STYRENE-ACRYLONITRILE AND AN OXYGENATED HYDROCARBON ADAPTEDTO PLASTICIZE SAID POLYMER, (C) A CATALYST ADAPTED FOR PROMOTING THEBURNING OF SAID AMMONIUM NITRATE, AND (D) A SALT OF NITRILOTRIACETICACID, SAID SALT BEING DERIVED FROM A NITROGEN BASE AND BEING USED IN ANAMOUNT AT LEAST SUFFICIENT TO IMPROVE THE STABILITY OF SAID COMPOSITIONWITH RESPECT TO GAS EVOLUTION.